Deep Reconstruction of Ni(OH)₂ via Magnetic-Field Regulation of Fe(OH)₃ Colloids for Efficient Oxygen Evolution...

Trace Fe impurities are ubiquitous in alkaline water electrolysis and commonly regarded as beneficial promoters for activating Ni-based oxygen evolution catalysts. However, the non-monotonic dependence of catalytic activity on Fe³⁺ concentration suggests a complex role beyond simple ionic promotion. Here, we show that Fe³⁺ predominantly exists as negatively charged Fe(OH)₃ colloids instead of free cations in alkaline electrolytes. Their accumulation at the positively biased electrode induces interfacial heterogeneities that limit the depth and uniformity of Ni(OH)₂ pre-catalyst reconstruction. We further demonstrate that an external magnetic field regulates the Fe(OH)₃ colloidal interface in a potential-dependent manner. During the initial activation stage, the magnetic field perturbs interfacial water matrix and weakens intermolecular interactions, thereby promoting Fe(OH)₃ interfacial adsorption and Fe incorporation, which accelerates the early-stage Ni(OH)₂ reconstruction. In the subsequent OER regime, the magnetic field sustains a dynamically restructured interface that enables thorough Ni oxidation. Consequently, the Ni(OH)₂ pre-catalysts evolve into Fe-rich and porous NiOOH structures with intrinsically higher catalytic activity, leading to markedly improved oxygen evolution performance. This work identifies Fe³⁺ colloidal chemistry as a previously overlooked factor governing Ni-based catalyst activation and establishes magnetic field modulation as an effective strategy to control interfacial reconstruction.